1. FIELD OF THE INVENTION
This invention relates generally to thermal-inkjet printing systems; and more particularly to apparatus and methods for ascertaining whether very small inkdrops have been ejected by thermal-inkjet pens. As will be described below, however, some such apparatus and methods have application to detecting other types of particles in other kinds of systems.
2. RELATED ART
Inkdrop detectors are useful in thermal-inkjet printing machines to ascertain whether an inkdrop exists, after an ink-ejection nozzle has been commanded to produce one. In this way these devices can verify that each ink-ejection nozzle or particular ink-ejection nozzles are operating properly, or can initiate corrective actions in event one or more nozzles is not operating properly, or can be used (as in coowned U.S. Pat. Nos. 4,922,268 of Osborne, and 4,922,270 of Cobbs, Haselby and Osborne) to calibrate the nozzle or pen position relative to other parts of the printing machine.
Coowned U.S. Pat. No. 4,835,435 of Yeung and Franz describes the art prior to 1988 in such printers and in other contexts. That patent, however--based on an assumption that optical detection systems necessarily require difficult, expensive optical alignments--proposes a detection system that is percussive, employing a piezoelectric membrane to develop an electrical signal in response to inkdrop impingement.
The resulting system works well for its intended purposes, but cannot be made sensitive enough for present and future thermal-inkjet printers. Indeed the size of inkdrops in such printers will decrease as this technology advances and the desired spatial resolution (number of addressable locations) becomes ever finer. Moreover, piezoelectric drop detectors have a drawback: ink must be cleaned from the sensitive surface at least occasionally.
Earlier optoelectronic drop detectors, such as that of Cobbs et al. mentioned above, similarly work well for droplets of 45 pL or more and in the '270 patent cover a field of view of about 0.15 by 21/2 cm (0.06 by 1 inch). Present-day pens, however, generate inkdrops about one order of magnitude smaller than that.
To maximize printer throughput with fine resolution it is desirable to make pens with a relatively very large number of nozzles; some modern pens accordingly have several hundred nozzles. In many or most designs the nozzles are arranged in two parallel columns, about a quarter-centimeter (tenth-inch) apart and each about 11/4 cm (one-half-inch) long.
The area circumscribing these columns is about 100,000 times the cross-section of an individual drop, but it is desirable that a detection device respond adequately to the presence of a single drop anywhere within such a large envelope--and be highly insensitive to the location of the drop. That is to say, for modern purposes an inkdrop detector must satisfy somewhat contradictory-seeming objectives of sensitivity high enough for a 41/2 pL drop but uniform response over a detection area corresponding to the shape and dimensions of the pen nozzle array.
These requirements are compounded by the desirability of providing a single detection device that can be used with a great variety of different pens and thus of nozzle arrays. Many different pens are designed into respective different printers, and in some cases two or more different pens are used in a single printer--even at the same time. For instance a printer may have one relatively small pen for firing only black ink, and a relatively larger pen with three nozzle sets or subarrays, each subarray loaded with ink of a different color respectively.
For modern small-volume inkdrops, especially in large detection envelopes over which uniformity of response is desired as mentioned above, drop-detection signals are extremely small. These signals are corrupted by noise from several sources--including the electromagnetic environment, stray light, and random variations of the drop-generation process--all adding to the difficulty of using inkdrop detectors to ascertain quickly and reliably whether individual nozzles are operating properly.
It can now be seen that prior inkdrop-detection devices, while serviceable for their design objectives, in very important ways leave room for refinement due to the more-demanding context of higher-resolution printers.